1. Field of the Invention
The present invention relates to a plating apparatus and a plating method for forming a uniform plating layer on a semiconductor substrate.
2. Background Of the Invention
FIG. 7A shows a face-up type plating apparatus of the prior art wherein surface of a wafer 101 is plated while being arranged to face upward, and FIG. 7B is an enlarged view of a sealing portion of the wafer 101. In the drawing, the numeral 1 denotes a wafer processing vessel, 2 denotes a plating solution nozzle, 2a denotes holes of a drain-board, 3 denotes a plating solution supply pipe, 4 denotes a plating solution discharge pipe, 5 denotes a drain pipe, 6 denotes a plating tank, 7 denotes a plating solution, 8 denotes an upper portion of the wafer processing vessel, 9 denotes a lower portion of the wafer processing vessel, 10 denotes a cathode contact, 11 denotes a sealing material, 12 denotes a nitrogen gas injection release, 14 denotes a mesh electrode,16 denotes an auxiliary sealing material and 101 denotes a wafer.
In the plating apparatus described above, the plating solution 7 supplied through the plating solution supply pipe 3 is discharged through the plating solution discharge pipe 4, and is circulated throughout the period of plating process. A specified voltage is applied across the mesh anode electrode 14 and the wafer 101 via the cathode contact 10, thereby to form a plated coating on the wafer 101 surface. In such a face-up type plating apparatus, because the wafer surface is arranged to face upward, deposition of air bubbles onto the wafer surface can be prevented and plated coating of better quality can be formed in comparison to the face-down plating method where the wafer is arranged to face downward.
FIG. 8 shows a distribution of a plated coating thickness over the surface of a 4xe2x80x3 wafer plated with Au in the above plating apparatus with a current density of 5 mA/cm2 for a plating time of 12 minutes, where a distance from the wafer edge is plotted along the axis of abscissa and plate coat thickness is plotted along the axis of ordinate. As is clear from FIG. 8, the plated coating thickness shows a W-shaped distribution which has a peak at the center of the wafer and increases toward the edge.
Through an investigation into the cause of such a distribution of the plated coating thickness, it was found that the distribution of the thickness is greatly affected by the distribution of the transported ions of the plating metal, which is determined by the flow velocity distribution of the plating solution, and the distribution of the electric field on the wafer surface. Specifically, in the plating apparatus described above, because a flow velocity of the plating solution is highest, and accordingly the transported quantity of ions of the plating metal is largest, at the center of the wafer which is located just below the plating solution supply pipe 3, a plated coating is formed with the largest thickness at the center, while the electric field is concentrated at the edge which leads to the plated coating being formed with the second largest thickness along the edge of the wafer.
On the other hand, a method may be used in which as the flow velocity of the plating solution 7 supplied through the plating solution supply pipe 3 is slower, thereby reducing the distribution of the flow velocity of the plating solution on the wafer surface. However, when such a method is used, the plating solution 7 becomes stagnant locally on the wafer surface, resulting in lower quality of the plating.
Thus, an object of the present invention is to provide a plating apparatus and a method of plating, which improve the uniformity of the plate coat thickness without changing the flow velocity of feeding the plating solution.
The present inventors have intensively studied. As a result, it has been found that unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution can be mitigated and uniform distribution of the plate coat thickness can be achieved over the wafer surface, by providing an aperture at the center of a meshed anode electrode of a plating apparatus thereby to obtain an electric field density distribution, between the meshed anode electrode and the wafer, that is lower in the central portion of the wafer than in the portion along the edge, thus completing the present invention.
That is, the present invention provides an anode electrode installed to oppose a wafer, whereon a plating coat is to be deposited, for generating a specified electric field distribution over the wafer surface, which is a meshed electrode capable of supplying plating solution and has an aperture at the center of the meshed electrode.
Because the meshed anode electrode has the aperture at the center thereof, an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge can be obtained by using the meshed electrode as the anode to form electric field between the electrode and the wafer.
Thus the flow velocity of the plating solution can be made lower in the central portion of the wafer than in the portion along the edge, making it possible to mitigate the unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution which has been a problem for the prior art, thus improving the uniformity of plate coat thickness over the wafer surface.
The meshed anode electrode may be an electrode made by weaving a thread-like material as shown in FIG. 2A, or an electrode made by punching holes through a sheet as shown in FIG. 2B.
The present invention also provides a plating apparatus which comprises, a plating tank wherein a wafer is placed so that the plating surface faces upward, plating solution supply means for causing the plating solution supplied from above onto the plating surface of the wafer at the center thereof to flow from the center of the plating surface of the wafer toward the periphery, and a meshed anode electrode installed to oppose a wafer for generating an electric field distribution by using the wafer as a cathode, wherein an aperture is made at the center of the meshed anode electrode to obtain an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge.
Because the plating apparatus of the present invention uses the meshed anode electrode which has the aperture at the center thereof, an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge can be obtained. As a result, unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution can be mitigated. That is, an increase in the plate coat thickness at the central portion of the wafer due to the flow velocity distribution of the plating solution can be suppressed by decreasing the electric field density in the central portion of the wafer, thereby making it possible to improve the uniformity of the plate coat thickness over the wafer surface.
The meshed anode electrode is a circular electrode having a diameter nearly equal the wafer diameter, and the aperture of the meshed anode electrode is preferably a circular aperture having a diameter of 40 to 80% of the wafer diameter.
This is because it is most preferable for improving the uniformity of the plate coat thickness to use the meshed anode electrode which has the same circular shape as the wafer and also has a circular aperture of diameter 40 to 80% of the wafer diameter.
The present invention also provides at method of plating the surface of a wafer, comprising the steps of causing the plating solution supplied onto the plated surface of the wafer to flow from the center of the plated surface of the wafer toward the periphery, generating an electric field between the wafer and the meshed anode electrode which is arranged to oppose the wafer, and generating such an electric field distribution that mitigates the unevenness in the plate coat thickness caused along the flow of plating solution by using the meshed anode electrode having the aperture at the central portion thereof.
By using such a method, a uniform plating coat can be obtained.
The present invention also provides a wafer for semiconductor devices provided with a plating layer plated by the above method, wherein the distribution of the plating layer thickness on the wafer is about 10%, more specially 5%.